This can be a problem with the file places.sqlite that stores the bookmarks and the history.
See:

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

The navigation buttons are not totally disabled. You just have to right click first and choose where you want to go. Of course, I prefer the old fashioned way when all you had to do was just click it and it will go back or forward one step. The refresh button is totally disabled though.

But for some reason, I had the pleasure of experiencing those buttons working a couple of times (no right clicking) but they were disabled again after I tested it by closing then re-opening firefox. For the life of me, I can't remember what I did to make those buttons work.